Hi claira,
Try the following:
Start .....type in ......cmd.......[do not hit Enter yet ]
Right click on cmd.exe that appears top left of screen > Run as Administrator
At the command prompt type ........sfc /scannow .....Hit Enter [ there is a space after sfc ]
This will check the system files.
If you have a Win7 disk you may be asked for it if there are any problems.